The efficiency of clipped pulse generation

ABSTRACT

The disclosed embodiments provide a resonant oscillator circuit. The resonant oscillator circuit includes a clipping mechanism configured to clip an output voltage of a signal pulse generated by the resonant oscillator circuit to a predefined constant level. The resonant oscillator circuit also includes a feedback path configured to return energy from the clipping mechanism to an input of the resonant oscillator circuit.

BACKGROUND

1. Field

The disclosed embodiments relate to resonant oscillator circuits. Morespecifically, the disclosed embodiments relate to techniques forimproving the efficiency of clipped pulse generation using resonantoscillator circuits.

2. Related Art

Oscillator circuits are commonly used to generate pulses in electricalsystems. However, commonly used oscillator circuits can consume asignificant amount of power, which is a disadvantage for systems inwhich reduced power consumption is desired, such as portable computingdevices. To solve this problem, “resonant oscillator circuits,” whichtransfer energy back and forth between inductive and capacitive circuitelements, can be used to generate clock pulses without dissipating asignificant amount of power. (For example, see U.S. Pat. No. 5,559,478,entitled “Highly Efficient, Complementary, Resonant Pulse Generation,”by inventor William C. Athas, filed 17 Jul. 1995.)

In the above-described resonant oscillator circuit, a stream of resonantpulses with a substantially sinusoidal shape is generated, and aswitching device is used to clamp the output voltage of the resonantoscillator circuit to ground on the falling edge of each pulse when thepulse potential reaches zero volts. Input voltage across the resonantoscillator circuit's inductor and the switching on of the switchingdevice may then reverse the inductor current from a negative value atthe end of a pulse to a positive value determined by the duration of theswitching device's on-time. In addition, the peak voltage of theresonant pulses may be determined by the amount of current in theresonant oscillator circuit's inductor at the time at which theswitching device is turned off.

It is also highly desirable to hold the amplitude (i.e., peak voltage)of the pulses to a predefined constant level, in much the same way thatthe low signal level of the pulses is held constant. Design constraintson the peak voltage (e.g., from the load circuit driven by the pulses)may be one reason for clipping the output pulses. For example, thepulses may be used to drive a set of metal-oxide-semiconductorfield-effect transistors (MOSFETs) with maximum operating voltageratings. The pulses may thus be clipped to the MOSFETs' rated maximumoperating voltage to mitigate degradation and/or reduced reliability inthe MOSFETs.

To clip the pulses, a nonlinear element such as a Zener diode may becoupled to the output of the resonant oscillator circuit. In turn, theamplitude of the pulses may be clipped to the Zener voltage of the Zenerdiode. However, the Zener diode may dissipate power that normallyoscillates between the load and the resonant oscillator circuit, thusincreasing the power consumption and reducing the efficiency of theresonant oscillator circuit.

Hence, what is needed is a mechanism for increasing the efficiency of aresonant oscillator circuit that generates clipped pulses.

SUMMARY

The disclosed embodiments provide a resonant oscillator circuit. Theresonant oscillator circuit includes a clipping mechanism configured toclip an output voltage of a signal pulse generated by the resonantoscillator circuit to a predefined constant level. The resonantoscillator circuit also includes a feedback path configured to returnenergy from the clipping mechanism to an input of the resonantoscillator circuit.

In some embodiments, the resonant oscillator circuit also includes acapacitor configured to store the energy from the clipping mechanismprior to returning the energy to the input along the feedback path.

In some embodiments, the clipping mechanism includes at least one of afirst diode and a metal-oxide-semiconductor field-effect transistor(MOSFET).

In some embodiments, the clipping mechanism is further configured torectify the signal pulse and enable charging of the capacitor using thesignal pulse.

In some embodiments, the feedback path includes at least one of a Zenerdiode and a buck regulator.

In some embodiments, the feedback path further includes a second diodeconfigured to prevent forward conduction in the Zener diode.

In some embodiments, the resonant oscillator circuit also includes

-   -   (i) an inductor coupled to the input and a capacitive load,        wherein the inductor is configured to act together with the        capacitive load to generate the signal pulse; and    -   (ii) a switching device configured to clamp the output voltage        to ground when the output voltage reaches zero volts.

In some embodiments, the capacitive load includes a gate of a MOSFET.The clipping mechanism may clip the output voltage to a maximumoperating voltage of the gate, thus allowing the MOSFET to be used inthe capacitive load in lieu of a larger, more expensive, and/or lessefficient MOSFET with a higher maximum operating voltage.

The disclosed embodiments also provide a pulse-generating circuit. Thepulse-generating circuit includes an inductor coupled to an inputvoltage and a capacitive load, wherein the inductor is configured to acttogether with the capacitive load to generate a signal pulse. Thepulse-generating circuit also includes a switching device configured toclamp an output voltage of the signal pulse to ground when the outputvoltage reaches zero volts. Finally, the pulse-generating circuitincludes a clipping mechanism configured to clip the output voltage to apredefined constant level, as well as a feedback path configured toreturn energy from the clipping mechanism to an input of thepulse-generating circuit.

In some embodiments, the pulse-generating circuit also includes acapacitor configured to store the energy from the clipping mechanismprior to returning the energy to the input along the feedback path.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a clipping mechanism in a resonant oscillator circuit inaccordance with the disclosed embodiments.

FIG. 2 shows a clipping mechanism in a resonant oscillator circuit inaccordance with the disclosed embodiments.

FIG. 3 shows a resonant oscillator circuit in accordance with thedisclosed embodiments.

FIG. 4 shows a voltage waveform for a resonant oscillator circuit inaccordance with the disclosed embodiments.

FIG. 5 shows a current waveform for a resonant oscillator circuit inaccordance with the disclosed embodiments.

FIG. 6 shows a flowchart illustrating the process of operating aresonant oscillator circuit in accordance with the disclosedembodiments.

In the figures, like reference numerals refer to the same figureelements.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the embodiments, and is provided in the contextof a particular application and its requirements. Various modificationsto the disclosed embodiments will be readily apparent to those skilledin the art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

Furthermore, methods and processes described herein can be included inhardware modules or apparatus. These modules or apparatus may include,but are not limited to, an application-specific integrated circuit(ASIC) chip, a field-programmable gate array (FPGA), a dedicated orshared processor that executes a particular software module or a pieceof code at a particular time, and/or other programmable-logic devicesnow known or later developed. When the hardware modules or apparatus areactivated, they perform the methods and processes included within them.

The disclosed embodiments provide a circuit for generating signalpulses, such as the resonant oscillator circuit described in theabove-referenced patent. The pulse-generating circuit and/or resonantoscillator circuit may generate signal pulses that are clipped at apredefined constant level with substantially flat regions at the minimumsignal level. Because the pulses exhibit increased duty cycle at themaximum voltage level, the signal pulses may more closely resemblesquare wave pulses than pulses from other resonant oscillator circuits.

To accomplish the clipping action at the high level, a non-linearclipping mechanism may be introduced to the circuit. As shown in FIG. 1,the clipping mechanism may be a diode 106. The cathode of diode 106 maybe tied to a voltage source V_(S), and the anode of diode 106 may betied to the signal voltage V_(OUT), which is the voltage across acapacitive load C_(LOAD) 104. V_(OUT) may be clipped because of designconstraints on the peak voltage of the circuit. For example, theamplitude of V_(OUT) may be held at a predefined constant level toenable the driving of components such as metal-oxide-semiconductorfield-effect-transistors (MOSFETs) in C_(LOAD) 104 with lower maximumoperating voltages than the peak amplitude of the signal pulses.

Assuming V_(D) is the forward voltage drop of diode 106 when diode 106conducts, when the signal voltage rises to V_(S)+V_(D), diode 106forward-biases and holds the signal voltage V_(OUT) constant while thecurrent of diode 106 decreases to zero. When the current of diode 106reaches zero, diode 106 becomes reverse-biased, and the voltage acrossC_(LOAD) 104 begins to decrease toward zero. When the voltage acrossC_(LOAD) 104 reaches zero, a switch M 114 connected to an inductor L 102of the circuit turns on, and the sequence repeats.

Alternatively, as shown in FIG. 2, a p-channel MOSFET 206 may be used toclip the output voltage of the signal pulse. The drain node of MOSFET206 is tied to the voltage source V_(S), the source node is tied to thesignal voltage V_(OUT), and the gate is tied to V_(S).

The action of MOSFET 206 is similar to that of diode 106 of FIG. 1. Whenthe gate-to-source voltage is more negative than one threshold dropV_(th) of MOSFET 206, MOSFET 206 conducts and clips the output voltageV_(OUT) while delivering energy to the voltage source V_(S). In anotherembodiment, the gate of MOSFET 206 is actively controlled by a circuitthat turns on MOSFET 206 when the voltage reaches V_(S) and turns offMOSFET 206 when the current flowing through the channel of MOSFET 206decreases to zero, such that MOSFET 206 behaves as an “ideal diode.”Once the voltage across a capacitive load C_(LOAD) 204 reaches zero, aswitch M 214 connected to an inductor L 202 of the circuit turns on, andthe sequence repeats.

A byproduct of the clipping action is the energy delivered through theclamping device to the voltage source V_(S). In addition, the efficiencyof a system utilizing the circuit may be improved by applying energyfrom clipping of the signal pulses to an input source elsewhere in thecircuit instead of dissipating the energy using the clipping mechanism.As shown in FIG. 3, an input voltage V_(dc) is connected through aninductor L₁ 302 to a capacitive load C_(LOAD1) 304. A switch M 314clamps the output voltage from L₁ 302 to ground on the falling edge ofeach signal pulse when the pulse potential reaches zero volts.

To clip the output voltage of the signal pulses, diode D₁ 306 (e.g., aSchottky diode) rectifies the output of L₁ 302 and allows a capacitor C₂308, which acts as the voltage source V_(S), to charge up to a maximumof one forward diode voltage drop of D₁ 306 below the peak voltage ofthe signal pulses. A feedback path 316 containing a feedback block FB310 may then pass power from C₂ 308 back to the input of the circuitthrough diode D₂ 312.

FB 310 may be implemented in a variety of ways. First, a simple solutionfor FB 310 may be a Zener diode with the cathode connected to C₂ 308 andthe anode connected to D₂ 312. The Zener diode regulates the voltage onC₂ 308 to the sum of the Zener voltage of the Zener diode, the voltagedrop across D₂ 312, and the pulse generator input voltage V_(dc). D₂ 312prevents forward conduction in the Zener diode along feedback path 316,so that the signal pulses are not limited to the pulse generator inputvoltage less the Zener diode forward voltage drop. In this solution, theamplitude of the clipped signal pulses may be controlled by selectingthe Zener voltage of the Zener diode.

Another solution for FB may be a buck regulator. The buck regulatordraws power from C₂ 308 at the rectified voltage and outputs power backto the pulse generator input at the regulated voltage. The clippedsignal pulse amplitude is not directly controlled and depends on theamount of power used by and passed through the buck regulator.

The rising and falling edges of the clipped signal waveform areasymmetric. As shown in FIG. 4, the original unclipped signal isrepresented by a dashed, symmetric waveform 402, while the clippedsignal is represented by a solid, asymmetric waveform 404. For both therising and falling edges, the shape of the edge is a sine wave centeredaround V_(dc). For the rising edge, the magnitude of the sine wave isgiven by V_(M):

V _(out)(θ)=V _(dc) +V _(M) sin θ  (1)

where

$\begin{matrix}{V_{M} = {I_{M}\sqrt{\frac{L}{C}}}} & (2)\end{matrix}$

I_(M) is the magnitude of the initial current set by the duration of theon-time for the first switching device (e.g., switch M 314 of FIG. 3).Throughout this discussion angular time is used: φ=ωt and ω=1√{squareroot over (LC)}. In addition, the falling edge is a sine wave ofamplitude V_(c):

V _(out)(θ)=V _(dc) +V _(c) sin(θπ/2)  (3)

Referring to FIG. 4, the waveform is divided into five regions:

φ₀: the rising edge from 0V to V_(dc)

φ₁: the rising edge from V_(dc) to V_(dc)+V_(c)

φ₂: the flat top region

φ₃: the falling edge from V_(dc)+V_(c) to V_(dc)

φ₄: the falling edge from V_(dc) to 0V

Phase angles for the rising edge may then be defined using Equation 1:

$\begin{matrix}{{\sin \; \varphi_{0}} = \frac{V_{dc}}{V_{M}}} & (4) \\{{\sin \; \varphi_{1}} = \frac{V_{c}}{V_{M}}} & (5)\end{matrix}$

Likewise, using Equation 3, phase angles for the falling edge may bedefined:

$\begin{matrix}{\varphi_{3} = {\pi/2}} & (6) \\{{\sin \; \varphi_{4}} = \frac{V_{dc}}{V_{c}}} & (7)\end{matrix}$

The phase angle for the flat top, φ₂, is determined by the current at φ₁flowing through the inductor divided by the voltage drop across theinductor, which is constant. When the voltage reaches V_(dc)+V_(c), thecurrent equals:

$\begin{matrix}{{I_{M}\cos \; \varphi_{1}} = {V_{M}\sqrt{\frac{C}{L}}\cos \; \varphi_{1}}} & (8)\end{matrix}$

The current then drops linearly during interval T₂ until the currentreaches zero amperes:

$\begin{matrix}{{\frac{V_{c}}{L}T_{2}} = {I_{M}\cos \; \varphi_{1}}} & (9)\end{matrix}$

The duration T₂ is then:

$\begin{matrix}{T_{2} = {{\frac{V_{M}}{V_{c}}\sqrt{LC}\cos \; \varphi_{1}} = {{\sqrt{LC}\frac{\cos \; \varphi_{1}}{\sin \; \varphi_{1}}} = {\sqrt{LC}\cot \; \varphi_{1}}}}} & (10)\end{matrix}$

Hence, there is a simple relationship between φ₂ and φ₁:

 ₂ =cotφ ₁  (11)

Note that the preceding analysis pertains to an ideal resonant circuit,neglecting non-idealities such as resistive losses.

The circuitry described above requires a pulse input to the firstswitching device. As in the case of U.S. Pat. No. 5,559,478, it ispossible to use two such circuits for self-oscillation, with the signaloutput of one circuit driving the gate input of the other circuit andvice versa.

FIG. 5 shows a current waveform 502 for the inductor (e.g., inductor L₁302 of FIG. 1) current of one of the circuits. At the terminus of thefalling edge, the current flowing through the inductor is:

$\begin{matrix}{I_{NEG} = {{- \sqrt{\frac{C}{L}}}V_{c}\cos \; \varphi_{4}}} & (12)\end{matrix}$

At the beginning of the rising edge from Equation 8, the current flowingthrough the inductor is:

$\begin{matrix}\begin{matrix}{I_{POS} = {\sqrt{\frac{C}{L}}V_{M}\cos \; \varphi_{0}}} \\{= {I_{NEG} + {V_{dc}\sqrt{\frac{C}{L}}( {\varphi_{0} + \varphi_{1} + \varphi_{2} + \varphi_{3} + \varphi_{4}} )}}}\end{matrix} & (13)\end{matrix}$

Divide all by V_(dc) and multiply by √{square root over (LC)}:

$\begin{matrix}{{{\cot \; \varphi_{0}} - {\cot \; \varphi_{1}} + {\cot \; \varphi_{4}}} = {\varphi_{0} + \varphi_{1} + \frac{\pi}{2} + \varphi_{4}}} & (14)\end{matrix}$

A useful identity is the following:

sin φ₁·sin φ₄=sin φ₀  (15)

In typical applications, the quantities V_(dc) and V_(c) are the knowninputs. Hence, the value of φ₄ is known and the equation above reducesto one independent variable:

$\begin{matrix}{\varphi_{0} = {{\cot \; \varphi_{0}} - \sqrt{( \frac{\sin \; \varphi_{4}}{\sin \; \varphi_{0}} )^{2} - 1} + {\cot \; \varphi_{4}} - {\sin^{- 1}( \frac{\sin \; \varphi_{0}}{\sin \; \varphi_{4}} )} - \frac{\pi}{4} - \varphi_{4}}} & (16)\end{matrix}$

The dependent variable is V_(M) and can be determined using iterativetechniques applied to Equation 14.

Of some interest is the extreme case when φ₂ is zero. If φ₂=0 then fromEquation 11:

$\begin{matrix}{\varphi_{2} = { 0\Rightarrow{\cot \; \varphi_{1}}  = { 0\Rightarrow\varphi_{1}  = \frac{\pi}{2}}}} & (17)\end{matrix}$

And from Equation 14:

V _(M) =V _(c)

sin φ₄=sin φ₀

φ₄=φ₀  (18)

Equation 14 then reduces to finding the fixed point for:

$\begin{matrix}{{\cot \; \varphi_{0}} = {\varphi_{0} + \frac{\pi}{2}}} & (19)\end{matrix}$

with the known solution of φ₀≈0.4579. φ₁ approaches zero as V_(c)approaches zero. Conversely, φ₂ approaches ∞. Hence, the frequency canbe slowed down to extremely slow rates simply by setting the clip pointslightly above V_(dc).

Of further interest is the case when V=V_(dc). The circuit would operateas a voltage doubler.

Finally, the energy delivered at potential V is the energy stored in theinductor at phase angle φ₁:

$\begin{matrix}{E_{out} = {{\frac{1}{2}{CV}_{M}^{2}\cos^{2}\varphi_{1}} = {\frac{1}{2}{C( {V_{M}^{2} - ( {V_{c} - V_{dc}} )^{2}} )}}}} & (20)\end{matrix}$

FIG. 6 shows a flowchart illustrating the process of operating aresonant oscillator circuit in accordance with the disclosedembodiments. In one or more embodiments, one or more of the steps may beomitted, repeated, and/or performed in a different order. Accordingly,the specific arrangement of steps shown in FIG. 6 should not beconstrued as limiting the scope of the embodiments.

First, an inductor (e.g., inductor L₁ 302 of FIG. 3) coupled to an input(e.g., V_(dc) of FIG. 3) of the resonant oscillator circuit and acapacitive load (e.g., capacitor C₂ 308 of FIG. 3) is used to generate asignal pulse (operation 602). Next, a switching device is used to clampthe output voltage of the signal pulse to ground when the output voltagereaches zero volts (operation 604). The signal pulse may thus beseparated from the next signal pulse by a substantially flat region atzero volts, as described in the above-referenced patent.

Next, a clipping mechanism is used to clip the output voltage to apredefined constant level (operation 606). The clipping mechanism may bea first diode (e.g., diode 106 of FIG. 1) and/or a MOSFET (e.g., MOSFET206 of FIG. 2). In addition, the clipping mechanism may allow the signalpulse to conform to design constraints on the peak voltage of theresonant oscillator circuit. For example, the clipping mechanism mayclip the output voltage to a maximum operating voltage of a gate of aMOSFET in the capacitive load, thus allowing the MOSFET to be used inthe capacitive load in lieu of a larger, more expensive, and/or lessefficient MOSFET with a higher maximum operating voltage.

Energy from the clipping mechanism is also stored (operation 608). Forexample, the energy may be stored by a capacitor (e.g., capacitor C₂ 308of FIG. 3) coupled to the clipping mechanism. The clipping mechanism(e.g., diode D₁ 306 of FIG. 3) may rectify the signal pulse prior toenabling charging of the capacitor using the signal pulse.

Finally, a feedback path is used to return energy from the clippingmechanism to the input (operation 610). The feedback path may include aZener diode and/or a buck regulator that regulate(s) voltage on thecapacitor and/or further clip(s) the signal pulse. The feedback path mayalso include a second diode that prevents forward conduction in theZener diode so that the signal pulse is not limited to the pulsegenerator input voltage less the Zener diode forward voltage drop.

The foregoing descriptions of various embodiments have been presentedonly for purposes of illustration and description. They are not intendedto be exhaustive or to limit the present invention to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention.

What is claimed is:
 1. A resonant oscillator circuit, comprising: aclipping mechanism configured to clip an output voltage of a signalpulse generated by the resonant oscillator circuit to a predefinedconstant level; and a feedback path configured to return energy from theclipping mechanism to an input of the resonant oscillator circuit. 2.The resonant oscillator circuit of claim 1, further comprising: acapacitor configured to store the energy from the clipping mechanismprior to returning the energy to the input along the feedback path. 3.The resonant oscillator circuit of claim 2, wherein the clippingmechanism comprises at least one of: a first diode; and ametal-oxide-semiconductor field-effect transistor (MOSFET).
 4. Theresonant oscillator circuit of claim 3, wherein the clipping mechanismis further configured to: rectify the signal pulse; and enable chargingof the capacitor using the signal pulse.
 5. The resonant oscillatorcircuit of claim 3, wherein the feedback path comprises at least one of:a Zener diode; and a buck regulator.
 6. The resonant oscillator circuitof claim 5, wherein the feedback path further comprises: a second diodeconfigured to prevent forward conduction in the Zener diode.
 7. Theresonant oscillator circuit of claim 1, further comprising: an inductorcoupled to the input and a capacitive load, wherein the inductor isconfigured to act together with the capacitive load to generate thesignal pulse; and a switching device configured to clamp the outputvoltage to ground when the output voltage reaches zero volts.
 8. Theresonant oscillator circuit of claim 7, wherein the capacitive loadcomprises a gate of a MOSFET.
 9. A method for operating a resonantoscillator circuit, comprising: using a clipping mechanism to clip anoutput voltage of a signal pulse generated by the resonant oscillatorcircuit to a predefined constant level; and using a feedback path toreturn energy from the clipping mechanism to an input of the resonantoscillator circuit.
 10. The method of claim 9, further comprising:storing the energy from the clipping mechanism prior to returning theenergy to the input along the feedback path.
 11. The method of claim 10,wherein the clipping mechanism comprises at least one of: a first diode;and a metal-oxide-semiconductor field-effect transistor (MOSFET). 12.The method of claim 11, wherein the clipping mechanism is furtherconfigured to: rectify the signal pulse; and enable charging of acapacitor using the signal pulse, wherein the capacitor is used to storethe energy from the clipping mechanism.
 13. The method of claim 11,wherein the feedback path comprises at least one of: a Zener diode; anda buck regulator.
 14. The method of claim 13, wherein the feedback pathfurther comprises: a second diode configured to prevent forwardconduction in the Zener diode.
 15. The method of claim 9, furthercomprising: using an inductor coupled to the input and a capacitive loadto generate the signal pulse; and using a switching device to clamp theoutput voltage to ground when the output voltage reaches zero volts. 16.The method of claim 15, wherein the capacitive load comprises a gate ofa MOSFET.
 17. A pulse-generating circuit, comprising: an inductorcoupled to an input voltage and a capacitive load, wherein the inductoris configured to act together with the capacitive load to generate asignal pulse; a switching device configured to clamp an output voltageof the signal pulse to ground when the output voltage reaches zerovolts; a clipping mechanism configured to clip the output voltage to apredefined constant level; and a feedback path configured to returnenergy from the clipping mechanism to an input of the pulse-generatingcircuit.
 18. The pulse-generating circuit of claim 17, furthercomprising: a capacitor configured to store the energy from the clippingmechanism prior to returning the energy to the input along the feedbackpath.
 19. The pulse-generating circuit of claim 18, wherein the clippingmechanism comprises at least one of: a first diode; and ametal-oxide-semiconductor field-effect transistor (MOSFET).
 20. Thepulse-generating circuit of claim 19, wherein the clipping mechanism isfurther configured to: rectify the signal pulse; and enable charging ofthe capacitor using the signal pulse.
 21. The pulse-generating circuitof claim 19, wherein the feedback path comprises at least one of: aZener diode; and a buck regulator.
 22. The pulse-generating circuit ofclaim 21, wherein the feedback path further comprises: a second diodeconfigured to prevent forward conduction in the Zener diode.
 23. Thepulse-generating circuit of claim 17, wherein the capacitive loadcomprises a gate of a MOSFET.